More than 40,000 people who work with coronavirus patients in Europe, Africa, Asia and South America will be involved, MORU co-principal investigator Professor Sir Nicholas White said.

"We really do not know if chloroquine or hydroxychloroquine are beneficial or harmful against Covid-19", he said, adding that the trial was the best way to find out.

It comes just days after US President Donald Trump told reporters that he had been taking the unproven anti-malarial drug to ward off the virus, which has killed almost 330,000 people worldwide.

Trump's aggressive push of hydroxychloroquine peaked in April when he told reporters:

"Take it - what have you go to lose?"

A study in the US showed the drug had no benefit in treating coronavirus patients and was also associated with a higher risk of death.

The Food and Drug Administration has warned hydroxychloroquine can cause heart rhythm problems and Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said there was no evidence the drug could prevent coronavirus.

The president - who has previously promoted coronavirus cures such as injecting disinfectant - cited "calls" and "good stories" as his evidence of hydroxychloroquine's success.

There have also been concerns raised in the US that the millions of people there who rely on it for treatment of lupus and rheumatoid arthritis could face more shortages because of an inevitable surge in demand for the drugs.

The UK government has said chloroquine and hydroxychloroquine are not licensed to treat or prevent COVID-19 and that the drugs should not be used outside clinical trials.

Prof Martin Llewelyn of Brighton and Sussex School, lead UK investigator for the MORU trial, said:

"If drugs as well tolerated as chloroquine and hydroxychloroquine could reduce the chances of catching COVID-19 this would be incredibly valuable.

"Like all drugs, chloroquine and hydroxychloroquine have side effects.

"But actually, used in the sort of doses that we will use them in the COPCOV trial, they're expected to be really, very safe."

Any adult who works in a UK healthcare facility and is delivering direct care to patients with proven or suspected COVID-19 can participate, as long as they have not been diagnosed with COVID-19 or an acute respiratory infection.

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